Recombinant Adeno-Associated Virus (rAAV) vectors have been instrumental in achieving restoration of vision in humans with Leber's congenital amaurosis due to RPE65 mutations and in a number of animal models (reviewed by Stieger, K. et al, Adeno-associated virus mediated gene therapy for retinal degenerative diseases. Methods Mol Biol 807, 179-218 (2011) which is incorporated by reference herein). The first retinal gene therapy clinical trials for retinal degenerative disease were initiated in humans in 2007 using AAV2 to deliver test the safety of therapeutic gene expression in children and adults with an early form of blindness called Leber's Congenital Amaurosis. Now, five years later, there are 7 Phase I/II gene therapy trials in progress involving inherited blindness. The first Phase III clinical trial initiated enrollment in the fourth quarter of 2012 (http://clinicaltrials.gov) and the results to date have been excellent. More than 125 individuals have already participated in retinal gene AAV-mediated gene therapy clinical trials and the safety records in these studies have been excellent.
Given the excellent safety and efficacy data relating to AAV-mediated retinal gene therapy and given the large number of inherited and acquired diseases that lead to blindness, there are many additional targets and strategies that are under consideration for application to the retina. One challenge, however, is to identify vectors which can efficiently and stably transduce a large variety of retinal cell types. Many of the rAAVs that have been characterized to date target retinal pigment epithelium (RPE) cells efficiently, and several of them target rod photoreceptors efficiently. One (AAV2) targets ganglion cells, a few target Muller cells (AAV2, AAV5, AAV8), and another (AAV9) targets cone photoreceptors. However, few AAVs have been found that target a majority of ocular cells with high efficiency.
In a normal eye, photoreceptors form the outermost layer of the retina. They convert light into electrical signals, which are sent to neurons in the retina's middle layer known as bipolar cells. Bipolar cells send visual information to the inner layer, made up of ganglion cells, which then connect to the brain via the optic nerve. In optogenetic therapy, artificial photoreceptors are constructed by gene delivery of light-activated channels or pumps to surviving cell types in the remaining retinal circuit. Because the bipolar cells are involved in processing visual signals, they have become the focus of attention from those interested in optogenetic therapies for the retina. However, so far, no recombinant virus has been identified which transduces bipolar cells of the retina.
Horizontal cells present an interesting target. Although, to date, no horizontal cell-specific genes are known that contribute to retinal disease, 40 genes are still unidentified from the 232 loci that are currently linked to retinal disease, and disease genes may yet be discovered in horizontal cells (https://sph.uth.edu/retnet/sum-dis.htm). Further, horizontal cells adjust photoreceptor output through feedback signals and are crucial to vision perception. More information is needed about horizontal cells to fully understand visual perception. Thus, the ability to express desired transgenes in these, and other ocular cell types, is needed.